Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
2001-04-25
2003-09-09
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S122000, C423S123000, C423S124000, C423S127000, C423S130000, C423S625000
Reexamination Certificate
active
06616902
ABSTRACT:
This invention relates to the improvement of the mineralogical and chemical composition of naturally occurring and synthetic alumina process feedstocks. The invention is particularly suited to the enhancement of boehmitic bauxites used in the production of alumina and alumina chemicals, especially by the Bayer process.
Embodiments of the present invention have the common feature of heating of the alumina process feedstock to bring about thermal dehydration and removal of organic carbon or conversion of organic carbon to a form which is not extractable in the aqueous phase digestion of the alumina process feedstock. Additional steps may be employed as will be described below.
The dominant technology for the extraction of refined alumina from alumina process feedstocks is the Bayer process. In the Bayer process alumina is extracted from alumina process feedstock (most frequently in the form of bauxite) by contacting the milled alumina process feedstock with hot caustic solution, generally under pressure, to dissolve alumina therefrom. If the alumina process feedstock contains mainly gibbsite (a mineral form of alumina trihydrate), extraction of alumina from the bauxite may be conducted using a caustic solution at a temperature generally in the range 100 to 175C. If the alumina process feedstock contains mainly boehmite, or diaspore (mineral forms of alumina monohydrate) higher temperatures, in the order of 200 to 300C. are generally required. The higher temperature digestion is required in these cases because the monohydrate forms act to cause instability of caustic solutions containing the high levels of dissolved alumina desired for subsequent processing unless there is a high degree of elimination of these forms by digestion at temperatures where such liquors will be stable. High temperature digestion comes with significant equipment cost disadvantages, in a much larger liquor heating and flashing system (e.g. 11 stages compared with 3) and in more expensive materials and specifications for construction. For mixed trihydrate and monohydrate forms, as is the case for many naturally occurring bauxites, a double digestion process, in which residues from a lower temperature first stage digest are further digested in a higher temperature second stage digest, may be used.
After digestion the digestion solid residue/pregnant caustic liquor mixture is brought back to atmospheric pressure by flashing to boil off water. The solid residue (usually referred to as red mud) is separated from the pregnant, caustic aluminate bearing liquor, usually by a combination of settling or filtration and washing, with both pregnant liquor and wash liquor clarified through pressure filters. The clarified combined liquor is fed to a precipitation circuit where it is cooled and seeded with solid particles of alumina trihydrate to induce precipitation of solid alumina trihydrate from the liquor. The resulting precipitation slurry is separated into a spent liquor stream and solids streams graded by particle size, by settling, cycloning or filtration, or combination of these processes. Coarse solids represent product, and are washed and transferred to a calcination stage where they are calcined to produce alumina. Intermediate and.fine solids are separately returned to the precipitation circuit, frequently after at least crude deliquoring, e.g. in cyclones or filters, for agglomeration and to provide seed.
The fine seed is normally washed prior to recycle to precipitation, either to remove solid phase oxalate precipitated with the alumina (which would interfere with the incorporation of the fine material into composite coarse particles in the precipitation process), or to remove organic compounds which would otherwise render the seed less active.
The spent liquor is returned to the digestion step, normally after some reconcentration by evaporation, where it is contacted with further milled alumina process feedstock.
The Bayer process has been used commercially for about 100 years and is well known to persons skilled in the art.
Alumina process feedstocks, particularly bauxites, include a range of impurities in addition to the hydrated forms of alumina. The main impurities are compounds of iron, titania and silica, which, while having various deleterious effects in the Bayer process, including on consumables such as flocculants, lime and caustic soda, and on scale formation and product quality, deport predominantly to the solid mud residue.
Despite its presence at only low levels in typical Bayer process feeds, extractable organic carbon (0.02% to 0.35%) is an impurity of major significance. Organic compounds, carbonates and oxalates derived from organic carbon in the feedstock have the capacity to accumulate in the circulating liquors, sequestering caustic soda which could otherwise have delivered alumina from digestion to precipitation, and therefore severely impacting on the productivity of the process. While carbonates and oxalates can be removed from the circuit by causticisation of various wash liquors or precipitates with lime, a reduction in the level of other organic carbon derivatives can only be achieved by either pressure oxidation (which comes with explosion hazards and generates large quantities of oxalate and carbonate which then must be removed), or bleeding off of caustic solutions, for either neutralisation and disposal (which is major economic burden through caustic make-up costs) or for concentration by evaporation followed by destruction by combustion (which has high energy and capital costs). Organic compounds also interfere with the precipitation process (by adsorption onto active sites on the seed, having a seed poisoning effect) and carry soda as a contaminant into the precipitated product. Oxalate derived from organic carbon is relatively insoluble, and can precipitate as sodium oxalate with the alumina trihydrate, interfering with product size, morphology and chemistry, and reducing resistance to particle attrition. Because these effects lead to the necessity to ensure that oxalate is not precipitated in the same precipitation tanks in which fine alumina is to be cemented into composite particles by the early portion of the precipitating alumina hydrate, and because oxalate stability above its solubility is a strong inverse function of liquor strength, the caustic strength available for carrying alumina is also limited in most alumina refineries by the input of oxalate precursors and oxalate generated by oxidation of other organics.
That is, organics in alumina process feeds are in large measure responsible for establishing the limits to productivity in the Bayer process, by setting the maximum level of soda in liquor, determining the extent to which this soda is sequestered from its useful purpose of delivering alumina, and acting as poisons for the precipitation process.
The impact of monohydrate alumina in alumina process feeds in driving the need for high temperature digestion has already been mentioned. Some other impacts of monohydrate alumina should also be mentioned. Digestion of alumina process feeds at high digestion temperatures results in side reactions (such as production of titania phases) which reduce digestion efficiency. For this reason lime addition is frequently made. The consumption rate of lime for this purpose and for causticisation and oxalate destruction is sufficient to justify the construction of dedicated lime kilns in many environments. Also, the digestion temperature is frequently limited by the pressures at which boilers can operate safely and effectively, which results in a greater limitation on liquor alumina concentration for high temperature digestion than for low temperature digestion, given the instability of high alumina concentration liquors in the presence of solid residues which still contain destabilising monohydrate alumina. Thus digestion of monohydrate alumina bearing alumina process feeds is naturally less productive than digestion of alumina bearing feeds with little or no monohydrate alumina. To make up for this shortcoming some alumi
Grocott Stephen
Hollitt Michael
Roe Gerard
Bos Steven
Comalco Aluminium Limited
Kerins John C.
Miles & Stockbridge P.C.
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